1. Field of the Invention
The present invention relates to the field of automatic transmission repair.
2. Prior Art
Automatic transmissions as are generally used in passenger vehicles usually incorporate a forward clutch assembly just aft of the pump which, when engaged, effectively connects the torque converter shaft to the output shaft for direct drive through the transmission. The direct clutch assembly is actuated by oil pressure delivered thereto through an opening in the hollow shaft-like extension of the pump body surrounding the torque converter shaft, hereinafter referred to as the pump shaft. The pump shaft is sealed with respect to the forward clutch housing by way of oil seal rings, usually three, which locate in cooperatively disposed seal ring grooves on the pump shaft to elastically expand outward against the clutch housing journal.
In most transmissions the oil seal rings sealing the pump shaft to the clutch housing are cast iron seal rings, being split in such a manner that the two ends of the split ring overlap so as to comprise a full circle in planform. The rings are installed in the seal ring groove on the pump shaft, with the pump assembly being slid thereover during the assembly operation to elastically compress the rings, whereby once assembled the rings are firmly elastically urged against the inner diameter of the clutch housing hub. In operation the rings are intended to rotate with the clutch housing rather than with pump shaft and, together with the seal ring grooves in the pump shaft, provide a highly restricted leakage path for oil under pressure therearound so as to allow the forward clutch to operate under the intended pressure when the transmission shifts into high gear (direct drive). In essence the seal rings rotate with the clutch housing and center themselves with the seal ring groove by hydrodynamic forces so as to not be subject to any appreciable wear. Accordingly, such cast iron rings, properly installed, rarely are a cause of automatic transmission failure.
In certain transmissions, specifically the automatic 200 transmission (hereinafter referred to as the A 200 transmission) manufactured by General Motors Corporation, the pump oil seal rings are Teflon seal rings which have good flexibility and high temperature capability. However, Teflon has a far lower modulus of elasticity than cast iron, and such rings are not elastically urged firmly against the inside diameter of the pump shaft. Therefore, such rings may allow fluid leakage thereby, reducing the hydraulic pressure holding the direct clutch in engagement when in direct drive. This, in turn, may allow the transmission to slip when in high gear, causing unnecessary wear and early failure of the transmission. Any slippage which is allowed becomes particularly troublesome when driving up a long, steep incline, as the slippage under such a load for any substantial length of time will cause excessive heating in the transmission, badly burning the oil, and leading to very early failure of the direct clutch assembly or other parts of the transmission. On the other hand, if the Teflon seal rings properly seat and stay properly positioned, no substantial leakage will occur and the rings will outlive the rest of the transmission. The problem, then, is one of uniformity and reliability, as the Teflon seal rings in many transmissions will never be a cause of difficulty, whereas in others, the rings may lead to a transmission failure within the first few thousand miles of the transmission's life, or within the first few thousand miles of a transmission overhaul.
Teflon rings are used in other portions of the A 200 transmission, which rings are typically replaced with cast iron rings whenever the transmission is rebuilt by independent transmission shops using rebuild kits supplied by the various parts kit suppliers. The pump oil seal rings, however, cannot be replaced with cast iron rings for the reason that the inside diameter of the front direct clutch bushing is smaller than the inside diameter of the pump shaft on which the seal rings must ride. As a result, the parts cannot be assembled using cast iron rings without the rings breaking when passing through the inside diameter of the front direct clutch bushing. As a consequence, it has been common to replace the A 200 transmissions with automatic 350 transmissions or, alternatively, because of the shortage of the 350 transmissions, to provide a warranty on rebuilt A 200 transmissions which is only a fraction of the warranty extended by independent rebuilders to other transmissions.